It has been reported that the salinity of an injection water can have a major impact on the recovery of hydrocarbons during waterfloods, with increased recovery resulting from the use of diluted brines (see, for example, “Labs Spin Out Oilfield Technologies”, American Oil & Gas Reporter, Vol 41, No. 7, July 1988, 105-108; “Effect of brine composition on recovery of Moutray crude oil by waterflooding”, Journal of Petroleum Science and Engineering 14 (1996), 159-168; and “Prospects of improved oil recovery related to wettability and brine composition”, Journal of Petroleum Science and Engineering 20 (1998) 267-276.
An established desalination method is known as “reverse osmosis” which in reality is a method of “ultra-filtration” through a membrane having minute micropores by applying a pressure differential on the seawater solution and across the membrane. However, problems associated with reverse osmosis include an undesirably low net extracted water product as a result of having to overcome and/or at least more than equal normal forward osmotic flow in the opposite direction, and membrane rupture as a result of the extremely high pressures necessary in reverse osmosis in conjunction with the thin and fragile membrane necessary to obtain or approach a near adequate flow rate of extracted water through the membrane.
An alternative desalination method is forward osmosis (also referred to as “direct osmosis”). Forward osmosis involves applying pressure to a first aqueous solution (for example, seawater) to facilitate the forward osmosis of water through an osmotic membrane into a second aqueous solution having a removable solute dissolved therein, to form a diluted solution, wherein the solute concentration of the second solution is greater than the solute concentration of the first solution and wherein the osmotic membrane has a sufficiently small pore size to exclude the solute of the first aqueous solution and the removable solute of the second aqueous solution from passing through the membrane. Thereafter, the removable solute is substantially removed from the dilute solution.
U.S. Pat. No. 3,171,799 relates to demineralizing water using a system in which two bodies of saline water, e.g. seawater, are separated by a semi-permeable membrane. A volatile solute is then added to one of the bodies of water. The addition of the volatile solute causes pure water, i.e. water containing substantially no salts, to migrate through the membrane from the solution which does not contain the volatile solute, thereby diluting the solutes, including the salts, in the latter solution. This dilution is continued until the desired concentration level of non-volatile salt is reached. The volatile solute is then removed. The process is said to function to produce an extremely dilute solution of non-volatile salts which has a sufficiently low concentration thereof to be suitable for drinking, production of steam etc. The concentration of non-volatile salts in a given solution may be decreased by providing a series of osmotic cells which will successively decrease the concentration of non-volatile salts. It is also said that the demineralization may be achieved using a system comprising a semipermeable membrane having on one side thereof a saline solution and on the other side thereof a fresh water solution containing a volatile solute. Suitable volatile solutes are said to include ammonia, sulfur dioxide, methyl acetate and acetonitrile.
U.S. Pat. No. 3,617,547 relates to a process applicable to the desalting of seawater or other salt-bearing water when a solvent in a solution having a solute difficult to separate from the solvent is extracted by passing the solvent through a permeable membrane to a solution comprising the solvent and a solute easily separated from the solvent. The solute (osmotic agent) for the recipient solution is easily separated from the solvent by precipitation leaving a substantially purer solvent product. For example, the osmotic agent may be a solute that is soluble at elevated temperatures and substantially less soluble at lower temperatures so that it precipitates and separates from solution. The membrane is permeable to the solvent and impermeable to the solute that is difficult to separate from the solvent and to osmotic agent. Typically, the solution containing the precipitated osmotic agent is transferred to a filter where the precipitated osmotic agent is separated from the solvent. As an alternative to removing the osmotic agent by precipitation at lower temperature, osmotic agents may be used that can be oxidized or reduced to a less soluble form and then removed by filtering and reconverting to the osmotic agent for reuse. Examples include cupric chloride that has a high solubility at room temperature and a low solubility when it is reduced to form cuprous chloride and ferrous acetate which is very soluble at room temperature but when oxidized it forms ferric basic acetate, which is insoluble.
It is also known that the injection water used in a waterflood should be compatible with the formation water. Thus, underground formation waters can contain resident ions such as barium (e.g. at a level of up to 3000 ppm, for example 50-500 ppm) and usually also calcium (e.g. at a level of up to 30,000 ppm, for example 1000-5000 ppm) both in the form of soluble chlorides, but also in the presence of sulphate ions, so the water is saturated with barium sulphate, and usually also calcium sulphate. This formation water can meet seawater, which can contain precipitate precursor ions such as soluble carbonate (e.g. at 100-5000 ppm) and sulphate (e.g. at 1000-3500 ppm). Mixing the two waters produces an aqueous supersaturated solution of barium sulphate and/or barium carbonate, and/or calcium sulphate and/or calcium carbonate, from which scale comprising these compounds deposits on surfaces. The meeting of the two waters can be in the formation, when seawater containing precipitate precursor ions is injected into the formation through an injection well at a distance from a production well to enhance oil recovery (i.e. a water flood treatment). The scaling may occur in the production well or downstream thereof e.g. in flow lines, or gas/liquid separators (for separating oil/water from gas) or in transportation pipelines leaving the gas/liquid separators. Carbonate scale may particularly form in the gas/liquid separator or downstream thereof, owing to reduction in gas pressure causing soluble calcium bicarbonate to form insoluble calcium carbonate.
U.S. Pat. No. 4,723,603 relates to a process for reducing or preventing plugging in fluid passageways of hydrocarbon-bearing formations and in production wells which is caused by the accumulation of insoluble salt precipitates therein. This objective is achieved by removing most or all of the precursor ions of the insoluble salt precipitates from an injection water at the surface before the water is injected into the formation. Thus, insufficient precursor ions are available to react with ions already present in the formation to form significant amounts of the insoluble salt precipitates. The precursor ions of the insoluble salt precipitates are removed by means of a reverse osmosis membrane. However, as discussed above a disadvantage of reverse osmosis systems is that they have to pressurize large amounts of water in the feed.
It has now been found that significant energy savings may be made by using forward osmosis to obtain a low salinity injection water. It has also been found that the membrane of the forward osmosis desalination plant may be tailored to be ion selective such that the permeate has a reduced concentration of precipitate precursor ions whilst ensuring that the total dissolved solids of the low salinity injection Water is in the desired range of 200 to 5000 ppm, preferably 500 to 5000 ppm. A further advantage of forward osmosis is that the membrane may be used to separate a first aqueous solution that is a high salinity water such as seawater from a second aqueous solution containing a removable solute also in a high salinity water such as seawater so that the second aqueous solution is diluted down to the desired total salinity through the migration of water from the first to the second aqueous solutions through a membrane. Yet a further advantage of using forward osmosis to obtain the low salinity injection water is that a portion of the solute that is employed to drive the forward osmosis process may be retained in the treated low salinity water provided that the total dissolved solids content of the injection water is the in desired range. It is preferred that the solute employed to drive the forward reverse osmosis does not act as a precipitate precursor ion in a “scaling” formation.